This invention relates to improved NPO dielectric ceramic compositions for manufacturing multi-layer capacitors that are particularly suitable for use in high frequency applications. More particularly, this invention relates to improved NPO low firing, temperature stable dielectric ceramic compositions which are capable of achieving very high Q values when employed in high frequency capacitor applications.
There are currently available in the marketplace many ceramic dielectric compositions which meet EIA (Electronic Industries Association) specifications for NPO temperature coefficient of capacitance (T.C.C.)--i.e., maximum allowable change in capacitance over a temperature range of -55.degree. C. to +125.degree. C. is .+-.30 p.p.m./.degree.C. These compositions may exhibit dielectric constants (K') in low (15-20), mid (60-70) or high (90-110) ranges. Some of these compositions constitute relatively high temperature (1250.degree.-1300.degree. C.) sintering formulations, such as for example as disclosed in U.S. Pat. No. 3,775,142, while others such as for example as disclosed in U.S. Pat. Nos. 4,394,456 and 4,500,942, the latter of which is assigned to the same assignee as the present application, constitute formulations disposed to be sintered at low temperatures in the range of 1120.degree. C. or less. As a general statement, high firing NPO formulations produce denser fired ceramic compositions when fabricated into multi-layer ceramic capacitors (MLCC's), such as for example the type illustrated in FIG. 2 of U.S. Pat. No. 4,379,319, which also is assigned to the same assignee as the present application, and the contents of which patent are hereby incorporated herein by reference.
Among the disadvantages of the above-noted prior art NPO formulations is that they generally require a larger energy input in order to sinter the ceramic; and this in turn necessitates the use of expensive internal electrode materials, such as palladium, platinum and gold, either separately or in combinations thereof. It is also well known to those skilled in the art that ceramic dielectric compositions having low dielectric constants (approximately 20) and high Q values (greater than 10,000) are primarily derived from MgTiO.sub.3 based formulations. These formulations are notoriously difficult to work with, especially in water based systems. This is due to the fact that free Mg++ ions present in these mixtures readily hydrate to form Mg(OH).sub.2, which in turn creates rheological problems when the material is made into the slip for tape casting during preparation of MLCC's. Because of the presence of the free Mg++ ions, there is often a need for increasing the amount of dispersant and/or the amount of the aqueous solvent (H.sub.2 O). This in turn often creates a green ceramic that possesses a lower green density than is desired. This lower green density translates into a lower fired density in the resultant multi-layer capacitor, which can have the effect of creating increased porosity and consequent degradation of the capacitor's electrical properties.
These same rheological problems are encountered when rare earth titanates, such as neodymium titanates, are employed as suggested by the above-noted U.S. Pat. No. 4,394,456. In such case the Nd+++ ions function in a manner similar to magnesium ions, and cause a fall off of the Q and Tf of a capacitor made therefrom, when the capacitor is operated at microwave frequencies.
Although MLCC's prepared from low firing NPO formulations can be sintered at or below 1120.degree. C., and therefore can employ lower cost internal electrode materials, nevertheless the low melting point glass components, which are added to the base ceramic formulation to enable the lower sintering temperatures, can induce porosity in the fired capacitor as the glass melts and diffuses through the base ceramic formulation. As in the case of the above-noted problem generated when free Mg++ ions are present in the formulation, any porosity resulting from the glass diffusion can lead to lower density of the fired ceramic, which in turn can lead to degradation of the electrical properties, such as reliability, insulation resistance (IR) and high frequency dissipation factor (df). Moreover, such degradation can also occur if the glass is not formulated correctly, since the glass components can react with certain of the components in the base ceramic formulation, thereby resulting in undesirable high loss phases which lead to a decrease in the overall Q of the sintered product.
There currently is a need for low fired ceramic dielectric compositions having moderate dielectric constants, excellent high frequency dissipation factor (df), and excellent temperature stability, in order to meet the increasing demand for MLCC's that are capable of maintaining enhanced properties in the 1 MHz to microwave frequency range. Historically operating frequencies of 1 KHz to 1 MHz were typical for MLCC's, but improvements in technology, especially in telecommunications, have led to new applications which extend into the microwave region. This is evidenced by the enormous growth in wireless communication devices, such as wireless telephones, personal paging systems, satellite navigation systems, etc. It is these systems which have created a demand for MLCC's and other passive components capable of operating in higher frequency ranges, for instance upwardly of 5 GHz.
Accordingly, it is the object of this invention to produce low fire, BaTiO.sub.3 based temperature stable dielectric compositions which are particularly suitable for producing MLCC's having dense fired micro-structures, and which are capable of being utilized in higher frequency applications.
Another object of this invention is to produce a low fire NPO dielectric composition having a low dielectric constant in a range of between 25 and 40, and which will allow for very tight control of the capacitance values within a temperature range of -55.degree. to 125.degree. C. of MLCC's made from such a composition.
A still further object of this invention is to produce a low fire NPO composition for producing a MLCC which will generate a dissipation factor of .ltoreq.0.010% at a test frequency of 1 MHz with an applied signal of 1.0 Vrms, and a Q in excess of 5000 at a test frequency of 5 GHz.
A further object of this invention is to produce from a low fire NPO composition MLCC's which will generate resistance values of &gt;10.times.10.sup.12 ohms at 25.degree. C. and insulation resistance values (IR) of &gt;100 Meg.OMEGA..multidot..mu.fd at 125.degree. C.
It is the object also of this invention to produce a low fire ceramic composition that obviates problems heretofore encountered when employing formulations containing MgTiO.sub.3 or rare earth oxides in the manufacture of MLCC's.
Other objects of this invention will be apparent hereinafter from the specification and from the recital of the appended claims.